To evaluate the long-term impact of the reduction of Philadelphia chromosome (Ph)-positive metaphases by treatment of chronic myelogenous leukaemia (CML) with interferon (IFN) alpha, we examined the outcome of 62 patients who had been enrolled between 1984 and 1990 into 2 IFN trials at our institution. As best cytogenetic response, 9 patients had achieved a complete remission and an additional 9 patients a partial remission. The remaining 44 patients had obtained either a minimal (n=29) or no cytogenetic response (n=15). Of the total of 62 patients, 9 were still on schedule and responsive to IFN in January 1995, including 7 patients in ongoing complete cytogenetic remission. The overall 5-year survival rate after a median follow-up from diagnosis of 51 months (range 3-102 months) was 62% and the median survival was reached at month 87. The effect of cytogenetic remission on survival was examined by "landmark" studies showing a significant survival advantage for patients with karyotype responses. In conclusion, in the patients studied, cytogenetic improvement was found to translate into improved survival expectancy. Long-term control by IFN alpha of CML, however, was restricted to a small minority of patients, predominantly to those attaining a complete suppression of the leukaemic cell clone as judged by cytogenetic criteria.